The study finds evidence of lasting immunity after mild or asymptomatic COVID-19 infection

New research by the Queen Mary University of London scientists has found evidence of protective immunity in people up to four months after mild or asymptomatic COVID-19.

The study published in Science Immunology analyzed antibody and T cell responses in 136 London healthcare workers who had mild or asymptomatic COVID-19 infection dating back to March 2020.

The team, involving researchers from Queen Mary, Imperial College London, and University College London, found after the analysis that 89 percent of healthcare workers carried neutralizing antibodies 16-18 weeks after infection.

The researchers also found most of the subjects had T cells capable of recognizing multiple distinct parts of the virus. However, the two responses did not always endure in harmony, with specific individuals showing T cell immunity but no evidence of antibodies, and vice versa.

Mr. Joseph Gibbons, a Postdoctoral Research Assistant at Queen Mary, said: “Our study of SARS-CoV-2 infection in healthcare workers from London hospitals reveals that four months after infection, around 90 percent of individuals have antibodies that block the virus. It’s even more encouraging that in 66 percent of healthcare workers, we see high levels of these protective antibodies and that this robust antibody response is complemented by T cells which we see reacting to various parts of the virus.”

“This is great news. It means that if you were infected, there is a good chance that you will have developed antibodies and T cells that may provide some protection if you reencounter the virus.”

Mismatched immune responses

Scientists worldwide have been working to understand how our immune system protects us against SARS-CoV-2 and how long this protection lasts right from the outset of the pandemic.

Much of this debate around protective immunity revolves around B cells’ different roles, making antibodies and T cells. These white blood cells work in several different ways to help protect from viruses, including direct killing.

In this study, the researchers show that a T cell response usually complemented protective antibody responses, but over half of the healthcare workers had mismatched antibody and T cell responses. They did not explicitly produce a T cell response to proteins in the outside layer of the SARS-CoV-2 virus.

They also discovered higher T cell responses those with the classic, defining symptoms of COVID-19. Simultaneously, asymptomatic infection resulted in a weaker T cell immunity than the symptomatic disease, but a similar response to the neutralizing antibodies.

Encouraging evidence

It is critical to understand how this careful choreography of immune responses works in people with mild or asymptomatic infection as they represent the largest infected group.

The new study also reassures vaccination efforts, suggesting that individuals carry antibody and T cell immunity to many parts of the virus, known as epitopes, even following mild infection. While new variants appear, the virus changes don’t necessarily occur within these epitopes, so, hopefully, the vast majority of immune recognition can probably continue undisturbed.

Dr. Corinna Pade, currently working as a Postdoctoral Research Scientist at Queen Mary, said: “Our study about asymptomatic and mild cases gives an optimistic insight into the persistence of immunity to SARS-CoV-2 after four months of infection. A striking number of around 90 percent of individuals have a joint force of strong antibodies that inhibits the virus from entering, coupled with T cell responses to various parts of the virus to interfere with its survival. This finding is important as mild, or even no symptoms of COVID-19 are prevalent and represent most of the community’s infections. Such ample immune responses also provide hope for the vaccines’ long-lasting efficacy.”

Áine McKnight, who is a Professor of Viral Pathology at the Blizard Institute at Queen Mary, appended: “Finally, here is the much-anticipated evidence of lasting antibody and T-cell immunity to SARS-CoV-2. Queen Mary played an important role in facilitating this study when many laboratories were shutting down at the pandemic’s outset. Our lab continued to be active and made fundamental scientific observations that contributed to this paper. We extend our support to the scientific effort against COVID-19 working with other London universities, NHS Health Trusts, and Public Health England to help control the pandemic.”